Add parallel test execution section to HACKING.rst

This commit adds a section to HACKING.rst to add some things to watch
out for now that we are running tempest in parallel by default. Part
of this documentation was taken from David Kranz's mailing list post:
http://lists.openstack.org/pipermail/openstack-dev/2013-August/014472.html

Change-Id: Ie688650fc528cf849c619e9afb64ba1f6b556ff8
Merge "Add parallel test execution section to HACKING.rst"